Laser welding using a laser beam having a high energy density is a high speed welding process with a small heat-affected zone. When there is a gap between the objects to be welded, however, the laser beam may pass through the gap, causing energy loss and resulting in defective welding.
To solve this problem, it has been suggested to use laser welding with a filler wire (see, Non-Patent Literature 1), or hybrid welding in which laser welding is combined with consumable or non-consumable electrode arc welding (see, Patent Literature 1). In the hybrid welding in which laser welding is combined with non-consumable electrode arc welding (TIG welding: tungsten inert gas welding), a filler wire is added to a welding area while a tungsten electrode, which is a TIG electrode, is vibrated, thereby obtaining a good weld (see, Patent Literature 2). When TIG welding is used, however, the tungsten electrode having a high melting point, may be burned out or consumed during welding, causing an unstable arc. Moreover, the consumed tungsten electrode may be incorporated into the weld metal, causing degradation in performance of the weld joint.
In the hybrid welding in which laser welding is combined with the consumable electrode arc welding (see, Patent Literatures 3 and 4), a wire can be added to a welding area without using a tungsten electrode, thereby achieving deep penetration welding. To achieve high speed machining and small melt width, in the hybrid welding using the consumable electrode arc welding, it is suggested to arrange arc welding electrodes in front of and behind the laser beam in the direction of welding (see, Patent Literature 5). The arc welding electrodes are MIG (metal inert gas) welding electrodes (wires). It is impossible, however, to adjust the feed speed of the wires and the MIG welding arc current independently of each other (see, Non-Patent Literature 2). In general, the arc current increases with increasing wire feed speed. Therefore, in the case of requiring a high deposited metal amount in the welding of sheet metals having a gap therebetween, a high wire feed speed is required, but this results in a high arc current, possibly causing burn-through.
FIGS. 16A and 16B are schematic diagrams showing conventional laser butt welding and conventional hybrid butt welding, respectively. FIG. 16A shows the conventional laser butt welding.
As shown in FIG. 16A, laser beam 1 is used to weld gap 3 between the butting surfaces of objects-to-be-welded 2. Welding objects-to-be-welded 2 together using laser beam 1 results in bead 4. When gap 3 is narrow or the welding speed is low, good bead 4 is obtained. When gap 3 is wide or the welding speed is high, laser beam 1 may pass through gap 3, causing bead 4 to have burn-through 5.
FIG. 16B shows the hybrid butt welding using conventional consumable electrode arc welding. As shown in FIG. 16B, arc 7 is generated between wire 6 and objects-to-be-welded 2, and melts the tip of wire 6, thereby forming droplet 8. Laser beam 1 and arc 7 together form molten weld pool 9 at the butt joint of objects-to-be-welded 2. Partial solidification of molten weld pool 9 results in bead 10. The hybrid welding shown in FIG. 16B allows wire 6 to be added, so that allowable gap 3 can be wider than in the laser welding of FIG. 16A. When gap 3 is larger than the allowable range or the welding speed is too high, however, bead 10 may be formed discontinuously with burn-through 11 at its center. In such a case, even when the feed amount of wire 6 is increased to fill gap 3, burn-through 11 still can occur. The reason for the occurrence of burn-through 11 is as follows. In general arc welding, wire 6 cannot be increased without being accompanied by an increase in the arc current. This increases heat input to objects-to-be-welded 2 and the size of molten weld pool 9 until it is too large to be held only by the surface tension generated around pool 9.